2-Methylhopanoid (2-MH) biomarkers provide a signature for cyanobacterial (cyano) contributions to organic matter (OM) in sedimentary rocks back to 2,700 Ma, although their occurrence is sporadic, unlike that of their hopanoid analogs. 2-MH are prevalent in sediment sequences deposited during oceanic anoxic events (OAEs) and are often associated with low d (super 15) N values indicative of nitrogen (N (sub 2) ) fixation by cyanos. Representative low d (super 15) N values occur throughout the early Aptian OAE (OAE1a; 120 Ma) at Shatsky Rise (ODP Site 1207) in the Pacific, while the proportions of 2-MH are higher during time intervals characterized by cooler sea surface temperatures and oxygenated waters, determined from the TEX (sub 86) proxy and biomarkers, respectively. In modern marine environments higher temperatures favor filamentous non-heterocystous cyanos as the dominant N (sub 2) -fixing organisms, and exclude heterocystous species, whereas unicellular cyanos are favored by low pO (sub 2) . Thus, the variations observed within OAE1a suggest that N (sub 2) -fixing cyano populations changed during these episodes of carbon cycle perturbation. The coupled response of 2-MH, cyano speciation, and O (sub 2) levels during OAE bears on their relationships in earlier times, especially during evolutionary diversification of cyanos on the early Earth. The proposition that 2-MH originated in heterocystous cyanos that perform N (sub 2) fixation helps explain their occurrence in the late Archean during a global-scale expansion of oxygenated habitats, and their scarcity in anoxic Paleoproterozoic sequences. The presence of isorenieratane in OAE sequences that contain 2-MH suggests changes in populations of green sulfur bacteria and cyanos as environmental conditions alternately favored anoxygenic and oxygenic photosynthesis, respectively. Perhaps changes in phytoplankton communities during OAE may re-enact the dynamic interchange of these organisms that first accompanied the transition to an oxygenated world. This interpretation requires biosynthesis of 2-MH by N (sub 2) -fixing heterocystous cyanos prior to the initial accumulation of atmospheric O (sub 2) , and the fact that these heterocystous cyano groups are phylogenetically more evolved members of the clade also argues against the interpretation that specific groups of cyanos post-date the Great Oxygenation Event.